We plan to measure the kinetics of sodium entry and exit across each of the cell borders in toad urinary bladder by a technique involving loading of the tissue with radioactive sodium followed by washout. Count rate data are then fitted to sums of exponentials, from which the fluxes and rate coefficients across the cell borders, and the size of the sodium pool, may be determined. In addition, the total sodium is determined chemically for subsequent comparison. We shall study the control of sodium entry across the mucosal border, which is partly sensitive to ouabain, by determining the effects of changes in calcium concentration, sodium concentration, and pH on the ouabain sensitive and insensitive portions of the influx and efflux processes. Electrophysiologic studies in urinary bladder and in gallbladder will be performed with intracellular and extracellular electrodes. These studies include the determination of the steady state resistances and EMF's of each element in the system. We will investigate the control of these resistances and EMF's by measuring them before and after inhibiting or stimulating the sodium pump. In addition, we will record intracellularly while making rapid changes in the bathing solutions in order to study the time-and voltage-dependent changes in the basolateral membrane potential following sudden alterations in the mucosal bathing medium. These change in potential are related to the driving force for sodium entry, and indicate the presence of a mechanism which controls a rheogenic pump at the basolateral border. We also plan to study the effects of changes in calcium concentration on the permselectivities of the outer border in toad urinary bladder, again using electrophysiologic techniques. In addition, we will study the electrical transients which occur during the passage of square current or voltage pulses, in order to determine te capacitances of the elements. Finally, we plan to study changes in the shunt and cellular pathways in Necturus gallbladder induced by sudden voltage clamps, in order to determine the control of these two pathways.